Wastewater treatment is primarily driven by the need to prevent pollution of lakes and rivers. When wastewater discharged to lakes and rivers contains too many organic compounds that act as food for biological growth or too many nutrients that feed the growth of the biological growth, that biological growth consumes available oxygen, in effect suffocating the wildlife normally found in the streams. Wastewater treatment seeks to eliminate this food (organics) and nutrients prior to discharging. As a final step, the wastewater is also disinfected (usually with chlorine) to prevent the spread of human pathogens (typically viruses and bacteria).
The organic compounds are the carbon-hydrogen compounds predominately formed as the result of biological activity. These compounds come in a wide variety of forms. In order to provide a measure of the amount of organic compounds, the industry has settled on the use of “Biochemical Oxygen Demand” or BOD (normally referred to as 5 day BOD, or BOD5), which simply means the quantity of oxygen consumed by a sample spiked with biological growth over a 5 day period. It is an indirect measure of the organic content.
The nutrients of interest are similar to those used to fertilize a lawn, phosphorus (P) and nitrogen (N). Nitrogen is typically present in the form of ammonia (NH3), which is broken down under aerobic conditions to nitrite and nitrate (NO2 and NO3). These in turn may be reduced to elemental nitrogen by treating under conditions without oxygen (called anoxic when NO3 is present). Normally wastewater is treated under oxic conditions, although certain high strength wastes (typically industrial) are most efficiently treated in the absence of any source of oxygen (air or NO3), a condition called anaerobic, or in the absence of oxygen in its elemental form, but NO2 and/or NO3 are present to provide a source of oxygen, a condition called anoxic.
Thus, the goal of wastewater treatment may be summarized as:
OffendingConstituentMeasured asTreated bySolidsTSSSettling out in a clarifier(Total Suspended Solids)OrganicsBODBiological oxic(Anaerobic for high strength)PhosphorusPBiological or by use ofchemicalsAmmoniaNH3Biological oxic(reduced to NO2/NO3)Nitrates/NO2/NO3Biological anoxicNitritesPathogens—Chlorine, Ultraviolet, OzoneThe means of treatment is to provide food (BOD), nutrients (P & N, both normally present in sufficient quantities for cell growth) and oxygen for the biological growth that does the bulk of the treatment. Wastewater treatment may be thought of as creating a comfortable home for the growth of beneficial biological growth to treat the wastes.
To accomplish this end, a wastewater treatment system is built with a biological unit. In biological units, two basic technologies have been used over the years. Originally in Britain, fixed film systems were developed in the 19th Century based on the principle of how rivers and streams become clean; that is, just grow the biological growth on a pile of rocks and trickle the wastewater over the rock pile. Thus, the trickling filter was born. Trickling filters proved to be reliable, and stable, although not particularly flexible in meeting modified treatment needs. Today modern trickling filters use plastic media with corrugated channels similar to those in corrugated cardboard.
In the 1930's a new technology was developed in which the biological growth was freely suspended or fluidized within a flooded tank, allowing for intimate mixing and contact between the waste material, the oxygen, and the biological growth. This technology is called activated sludge, because the tank is continually reseeded or “activated” with a biological growth settled out in a subsequent clarifier and returned to the head end of the basin.
Thus, there remains a need for a new and improved wastewater treatment system based on Integrated Fixed Film Activated Sludge (IFAS) technology that resolves old technical issues. The new and improved wastewater treatment system combines the two technologies to increase the overall concentration of a biological growth available for treatment. The combination creates a system with the stability and robustness of trickling filters and the operational flexibility of activated sludge.